One known type of filter element for filtering fluid such as hydraulic or lube oil comprises a sheet of filter material folded to form a plurality of parallel pleats or folds. The side edges of the sheet are brought together so that the sheet has a cylindrical configuration, with the pleats of the sheet extending in the axial or longitudinal direction. The side edges of the sheet are then joined together such as with adhesive, stitching or other means, to retain the filter material in the cylindrical form.
Such a pleated media has certain advantages over non-pleated media, as the pleats provide a large surface area in contact with the fluid to be filtered, which increases the dirt holding capacity and minimizes the pressure loss.
The pleated filter media can be coreless, i.e., self-supporting, and received over a cylindrical perforated support tube integral with the filter housing when the element is located in the housing; or can include a cylindrical perforated core integral with the element. End caps are typically received on either end of the filter element with appropriate adhesive being applied between the end caps and the media. One of the end caps has a central opening such that fluid passing radially through the filter media is allowed to flow through the opening to an outlet passage in the housing. Fluid to be filtered typically passes radially inward through the filter media and then outward through the opening in the end cap, as this is the most advantageous flow direction for the efficient filtering of fluid.
It is important that the pleats of the filter media be able to withstand the pressure of fluid flowing through the media. If a significant pressure occurs, the pleats can become deformed, and fold-over or bunch against one another. This is not desirable, as the surface area of the pleats available for filtration is reduced, which reduces the useful life of the element. One solution is to impart the filter media with resin, which is then cured to give the filter media a certain amount of rigidity. However, solely a resin-cured media cannot withstand the pressure drops occurring across the media in many applications. The resin curing also adds manufacturing steps, which increases the overall cost of the element. Alternatively (or in addition to the resin curing), a thin, epoxy-covered steel mesh can be layered with the media when the media is pleated. However, this also increases the manufacturing steps for fabricating the filter element, and increases the cost of the element. Steel mesh also does not lend itself to easy disposal in landfills, that is, it is not easily incineratable, which raises environmental issues.
A number of techniques have been proposed to directly support the pleats of the filter media. Humbert, Jr., U.S. Pat. No. 3,306,794, for example, shows a pleated filter media surrounding a central core, where an outer perforated paper layer is wrapped around the pleats, and is attached to the pleat peaks with globules or droplets of adhesive cement. The side edges of the paper layer are overlapped and adhesively attached together. It is also known to wrap a spiral support layer around the pleats, and to attach the spiral wrap to the peaks of the pleats with hot melt adhesive. It is also believed that fusion bonding has been used to attach an outer support wrap to an inner filter media.
Another technique is shown in Hurst, U.S. Pat. No. 4,956,089 where pleated media is located between inner and outer support cages. Each of the support cages consists of a gridded tube of thermoplastic material such as polypropylene. Degen, U.S. Pat. No. 4,693,985; Asher et al., U.S. Pat. No. 5,824,232; and Miller, U.S. Pat. No. 4,609,465, show similar structures, where an outer support weave, mesh or cage is formed from material such as fluoropolymers, halogenated polymers, polyesters, or polyolefins such as polypropylene and polyethylene. Applicants believe that monofilament fabric of high density polymer is typically used for the support cages. As should be appreciated, such materials provide relatively rigid support cages.
In fact, it is believed that prior art support structures for pleated medias have either been relatively rigid, cage-like structures which are intended to provide support for a pleated media between the cages; or have been formed of a more flexible material which is mechanically or chemically attached to the pleats of the media. While these techniques might provide sufficient structural integrity for pleated media in many applications, it is believed that these techniques can have certain drawbacks. Providing rigid inner and outer cages, for example, requires relatively expensive polymers or other materials, and expensive manufacturing steps to form the cages. The manufacturing tolerances of the cages also have to be carefully controlled so as to fit closely over the pleats. Further, while the cages purport to provide structural integrity for the element, it is believed that such structural integrity is mainly in the axial direction, i.e., to prevent crushing of the element. The pleats are still generally free to move within the cages. This can still raise issues of fold-over and bunching of the pleats during use.
On the other hand, adhesively or otherwise bonding the pleat peaks to an outer wrap also requires additional manufacturing steps, which also increases the overall cost of the filter element. There can also be issues of compatibility of the fluid to be filtered with the adhesive, and the adhesive bonds breaking during use. This can be unacceptable in many applications.
As such, Applicant believes there is a demand in the industry for a pleated filter element with an outer support structure which is easily located over the pleats, and which prevents the pleats from moving relative to each other during use, but which does not require additional and expensive mechanical or chemical bonding of the support structure to the pleats.